taking for granted; rash conclusion
Used when someone jumps to a conclusion or assumes something without proper evidence, often in the phrase 一人腹を決める or similar constructions. Somewhat literary or old-fashioned.
He jumped to the rash conclusion that I had lied.
You must not judge things by taking them for granted.
Compound of 一人 (ひとり, 'one person') and 腹 (はら, 'belly/mind'). The exact derivation is uncertain, but it likely refers to making up one's mind alone, leading to a hasty or self-centered conclusion.